The invention is in the field of communication technology and more particularly, relates to methods for providing improved capacity of the communication channels.
Quadrature Amplitude Modulation (QAM) is used for transmitting information by transmitting two modulated carrier waves over a channel: for example, electromagnetic waves in space or over a wire or light waves over an optical fiber (note that the light waves are also electromagnetic waves). These two carrier waves (in-phase carrier and quadrature carrier) have the same carrier frequency and a phase difference of 90°.
The modulation of the in-phase and quadrature carrier waves at each time may be represented as a point on an I-Q plane (which may be considered a complex plane for various purposes). By selecting several points or regions on the I-Q plane and modulating the carrier waves so that they end up at different moments in different points or within different regions on the I-Q plane, binary or digital information may be represented and transmitted.
Such sets of points or regions are called constellations. Each point (or a complex number) in a constellation may be called a symbol and may represent one or several bits.
For signal constellations of the type QAM2N0, where QAM is Quadrature Amplitude Modulation and N0 is the efficiency of the signal constellation in bits per 1 Hz of bandwidth, N0 may be any positive integer; for QAM2N0, 2N0 is the number of points in the constellation.
Quadrature Amplitude Modulation of the type QAM2N0 may be used in different varieties and under different names. For binary phase-shift keying (BPSK), N0 is 1; for quadrature phase-shift keying (QPSK), N0 is 2; for 8PSK (phase-shift keying), N0 is 3; for QAM16 and for asymmetric phase-shift keying (APSK16), N0 is 4; etc.
The ideal spectral efficiency for communication channels deploying these modulation types in combination with LDPC+BCH coding is introduced in European Standard for Digital Video Broadcasting (DVB) ETSI EN 302 307 V1.1.1 (2005-03), page 33.
In practice, the transmission efficiency, for example, in modems such as DVB-S2 modems, is somewhat inferior due to impairments including the effects of linear and non-linear distortion (see, for example, CDM-710 Broadcast Satellite Modem, Installation and Operation Manual, Part Number MN/CDM710.IOM Revision 7, June 2007, Table 8-2, p. 8-8).